In mammals, efficient system-wide adaptive immune protection is mediated by the spleen, a complex system of lymph nodes, and mucosal associated lymphoid tissues (MALTs) strategically located throughout the body. However, the lymphatic system is a recent adaptation emerging in endotherms. Thus, how basal jawed vertebrates (cartilaginous fishes, bony fishes, amphibians, reptiles) that only possess a spleen can achieve effective immune surveillance in their absence is unclear. To investigate this question, I studied the adaptive immune system of the nurse shark, a member of the class Chondrichthyes (shark, rays, skates, chimeras), which is the oldest extant jawed-vertebrate lineage to possess adaptive immunity. In this dissertation, I demonstrated that the nurse shark utilizes the pancreas as a secondary lymphoid organ (SLO) in conjunction with the spleen to provide antigen specific protection against the periphery. I showed that this protection is facilitated by pancreatic B cell follicles, that exhibit hallmarks of B cell selection such as native antigen presentation, somatic hypermutation of B cells, and T cell help. Importantly, pancreatic B cell follicles were present in an unimmunized animal, showing they are natural and persistent features of the nurse shark immune system. Further, I also investigated how the nurse shark conducts adaptive immune protection at the mucosa by functionally characterizing organized MALT in the nurse shark spiral valve. I showed that spiral valve MALT contains AID⁺ B cell follicles resembling mammalian Peyer’s patches, along with effector cells in the lamina propria and villi, including γδ T cells and putative plasma cells. Within spiral valve mucus I identified secreted antibody that coated the microbiota, demonstrating an ancient function of mucosal immunity. In total, this dissertation indicates that extra-splenic lymphoid tissue was present early in the evolution of adaptive immunity and was a necessary feature to facilitate protection against peripheral and mucosal antigens.